Integrated circuits are required to operate at increasingly lower power supply voltages and increasingly higher speeds. Integrated circuits are often used in applications where low power consumption and high speed are important, such as in a battery powered computer, or the like. An effective method for decreasing power consumption is to use a lower power supply voltage (for example, 3.3 volts).
There are several different logic families from which to choose when designing an integrated circuit. Among the different logic families are ECL and BINMOS. ECL (emitter-coupled logic) circuits employ bipolar transistors that operate in the active operating region and typically provide very high switching speeds, however, ECL circuits consume a large amount of power. The difference between an ECL logic high and an ECL logic low is only about one base-emitter diode voltage drop (V.sub.BE). On the other hand, a BINMOS logic high is about one V.sub.BE below the positive power supply voltage, and a BINMOS logic low is approximately equal to the negative power supply voltage. BINMOS logic is usually implemented using BICMOS technology. A BICMOS circuit combines bipolar transistors with CMOS (complementary metal-oxide-semiconductor) transistors on the same integrated circuit. Generally, the bipolar transistors of a BICMOS circuit provide the advantage of high speed and high drive capability, while the CMOS transistors provide the advantage of reduced power consumption.
In order to achieve compatibility between the two different logic families, a level conversion circuit converts, or translates, a logic signal from ECL logic levels to BINMOS logic levels. A level conversion circuit should not cause excessive delay or consume a large amount of power. A conventional BICMOS level converter has a differential amplifier, emitter-follower input transistors for receiving an ECL level logic signal, and emitter-follower output transistors. The differential amplifier includes NPN transistors having their emitters coupled together, and a resistor coupled between each of the collectors and a positive power supply voltage terminal. N-channel MOSFET current sources are used to supply a relatively constant current.
The conventional BICMOS level converter, designed to operate with a power supply voltage of 5.0 volts, may not work reliability with a 3.3 volt power supply. When a 3.3 volt power supply is used instead of the 5.0 volt power supply, the operating margins of some of the transistors in the level converter circuit may be reduced. The margins are reduced in the conventional BICMOS level converter because a V.sub.BE voltage drop across a forward biased bipolar transistor is independent of the power supply voltage, and remains relatively constant. Therefore, when the power supply voltage is reduced, there may not be sufficient voltage range to allow for all of the V.sub.BE voltage drops without adversely affecting the operation of the circuit. Also, the integrated circuit may be more vulnerable to noise and to fluctuations in power supply voltages. In addition, the conventional BICMOS level converter may be considerably slower when operated at a lower power supply voltage, because the bipolar transistors of the ECL portion of the level converter may now be operating in the saturation operating region. If the bipolar transistors are allowed to operate in saturation, increased switching time, and an undesirable reduction in speed may result.